Persistent Pulmonary Hypertension of the newborn (PPHN) contributes significantly to neonatal morbidity and mortality, but its pathogenesis, pathophysiology and treatment are poorly understood. Characterized by the presence of severe hypertension, abnormal vasoreactivity and structural remodeling of the pulmonary circulation, PHHN represents the inability to achieve or sustain the decline in pulmonary vascular resistance at birth. Despite multiple interventions, current therapy for PPHN is often not successful, lacks specificity and can contribute to further lung injury. We hypothesize that endothelial-derived products contribute significantly to regulation of normal perinatal pulmonary vascular tone, and that vascular injury alters endothelial cell function, leading to sustained pulmonary hypertension and abnormal vasoreactivity due to diminished release of endothelial-derived dilators or increased endothelin production. To determine the role of altered endothelial cell function in the pathophysiology and the response to vasodilator therapy of PPHN, we propose a series of parallel in vivo and in vitro experiments in perinatal sheep and clinical studies of human PPHN. Our specific aims in animal studies are to study: 1) the roles of endothelium-derived relaxing (EDRF) and hyperpolarization (EDHF) factors and endothelin in regulation of normal perinatal pulmonary vascular tone and reactivity; and 2) if exposure of the developing pulmonary circulation to adverse stimuli, including acute asphyxia and chronic hypertension, impairs endothelial function, leading to failure of the transition of the pulmonary circulation at birth. In human newborns with PPHN, we will: 1) prospectively measure pulmonary artery pressure in neonates with severe pulmonary hypertension and compare clinical responses to intrapulmonary infusions of endothelium-dependent and independent dilators; 2) study the role of endothelin in its pathophysiology; and 3) examine EDRF activity in vitro of small pulmonary arteries from newborns dying with PPHN. The overall goals of these studies is to improve understanding pf mechanisms regulating vascular tone in the normal and abnormal perinatal pulmonary circulation, and to develop new therapeutic strategies in the clinical management of PPHN.